1. Field of the Invention
The invention relates to a process for the selective monoaddition of an olefin or acetylene to a siloxane which contains two reactive Si--H bonds to produce a product in which only one of the two Si--H functions has added across the olefin or acetylene. The invention also relates to a process for making unsymmetrical siloxanes from symmetrical dihydrosiloxanes and to the products of these processes.
2. Information Disclosure
A major goal in organic and polymer chemistry is to devise methodologies which would enable one to carry out highly specific reactions at one reactive site within a molecule, while at the same time leaving other potentially reactive sites unreacted. Of particular interest to polymer chemistry is the synthesis of molecules which have two different reactive sites for polymerization. Such molecules are highly sought after, since they can be used to prepare a wide variety of reactive intermediates, oligomers and polymers.
The hydrosilylation reaction is well known in the chemical literature and consists of the addition of a Si--H compound across a double or triple bond. For example, the addition of a trialkyl silane to an olefinic double bond as depicted in the equation shown below, results in the formation of a new carbon-silicon bond. ##STR2## Traditionally, platinum, palladium, rhodium, cobalt, iridium and iron complexes have been used as catalysts in this reaction. The hydrosilylation reaction is an exceptionally useful reaction which has been employed for the synthesis of a wide variety of interesting intermediates. For example, Lien et al. (U.S. Pat. No. 4,156,046) and Anderson et al. (U.S. Pat. No. 4,304,806) disclose the condensation of trialkoxysilanes with epoxides containing double bonds to obtain compounds which can be used to produce hard, protective coatings for films, negatives and optical discs. ##STR3##
Umpleby (U.S. Pat. No. 4,803,244) discloses the hydrosilylation of an unsaturated elastomer containing multiple carbon-carbon double bonds with a poly(hydrosilane) in the presence of a saturated thermoplastic polymer to produce a thermoplastic elastomer useful for the manufacture of pipes, tubing and fiber optic, wire and cable coating.
Takayanagi et al. (U.S. Pat. No. 5,096,990) disclose the reaction of a divinylpolysiloxane with a dihydropolysiloxane and a poly(hydrogensilane) in the presence of a platinum catalyst to produce resin compositions suitable for coating semiconductor chips.
These and other references which discuss the use of poly(hydrosilanes) for hydrosilylation all produce polysilylated products in which no attempt is made to induce selective reaction with a single Si--H bond in a molecule having multiple Si--H bonds. On the contrary, the usual goal is to "olefinate" as high a proportion of Si--H groups as possible.
Nakos et al. (European Appln 388,005) disclose the selective hydrosilylation of vinyl-functional norbornenes. As in the cases described above, all the Si--H bonds are reacted; however, in the case of the Nakos disclosure, subsets of carbon-carbon double bonds are selectively hydrosilylated.
Nagashima et al. [Organometallics 8, 2495-2496, (1989)] disclose that 1,2-bis (dimethylsilyl) ethane undergoes anomalously rapid hydrosilylation to form a monohydrosilyladduct and that the addition of a second mole proceeds at a normal rate. They attributed this phenomenon to the formation of a cyclic bidentate ligand with the catalyst: ##STR4## and supported this explanation with the observation that when two more methylenes were interposed [i.e. 1,4-bis(dimethylsilyl)butane], there was no acceleration of the initial addition. Compounds having more than three atoms between the silicons would not be expected to exhibit any selectivity because the formation of the requisite cyclic structure is not favored.
There thus remains a need for a method for carrying out selective olefination of a single Si--H bond in siloxane polymers containing two such bonds. This would allow easy access to a whole host of differentially functionalized silanes that could then be used as intermediates for the synthesis of useful polymers.